Method for gas separation by phase enhanced gas-liquid absorption

ABSTRACT

A new method called phase enhanced gas-liquid absorption has been developed. By addition of organic phase into the absorption system of gas aqueous phase, the absorption rate was increased significantly. Absorption rate was enhanced by organic phase.  
     This invention provides a new technology for improving the efficiency of traditional gas absorption. By reducing mass transfer resistance in absorption process, this technology is able to lead to a more efficient, low cost absorption process and significantly reduce constraints and costs.

[0001] No cross-reference to related applications.

[0002] Not Federally sponsored research or development

FIELD OF THE INVENTION

[0003] This invention relates to a method for gas separation. Inparticular, this invention relates to a new method for gas separation byphase enhanced gas-liquid absorption.

BACKGROUND OF THE INVENTION

[0004] Up to now, absorption is still a powerful tool for the gasseparation and purification [Arthur L. Kohl & Fred C. Riesenfeld; GasPurification. 4^(th) Edition; Gulf Publishing Company; 1985]. However,current processes for the enhancement of gas liquid absorption rate arestill limited to the chemical reactions which occurs in the liquid phasebetween gas component and liquid solution. The drawback is obvious. Thereaction will cause difficulty for the release of the gas componentsfrom liquid. The chemical reaction brings two benefits for absorption.One is the increase of carrying capacity for gas components. Another isthe reduce of mass transfer resistance or increase of mass transfercoefficient. Both factors contribute to the increase of absorption rate.Currently, most popularly used absorbents for carbon dioxide separationinclude alkanolamines, alkaline salt and their modified form.

[0005] Aqueous solution of alkanolamines has been used widely toseparate carbon dioxide from other gas to meet very low treated gasspecification. The major commercial interested amines, such as,monoethanolamine (MEA), diethanolamine (DEA) etc. can undergo sidereactions with carbon dioxide and form various kinds of degradationcompounds. These compounds reduce performance of the solvent and causemore energy consumption and corrosion [R. H. Niswander, D. J. Edwards,M. S. DuPart, and J. P. Tse, Separation science and technology, 28(1-3),pp. 565-578 (1993)].

[0006] Another popular method for carbon dioxide separation is alkalinesalt solution. Sodium carbonate and potassium carbonate are the mostcommon used material. The process can be divided into two types based onabsorption temperature, ambient temperature (70-100° F.) and elevatedtemperature (approximating regeneration temperature). At ambienttemperature, the absorption is very slow, which causes low efficiency ofcarbon dioxide recovery and the high steam requirement for regeneration.Absorption at elevated temperature, such as Benfield process, overcomesome of the disadvantage of ambient temperature process. By increasingtemperature, absorption rate and gas holding capacity are increased.Several modifications have been developed to accelerate the absorptionrate of carbon dioxide. In these process, activators or promoters areadded into carbonate solution, such as New Activated Benfield Process,The Catacarb Process, The Glammarco-Vetrocoke Process etc. As reported,activated solution is able to reduce the operating costs [The BenfieldCorporation. 1971. The way to low cost scrubbing of CO₂ and H₂S fromindustrial gas].

[0007] Sartori and Savage [Arthur L. Kohl & Fred C. Riesenfeld; GasPurification. 4^(th) Edition; Gulf Publishing Company; 1985, p 235]compared absorption rates and vapor-liquid equilibria of CO₂ inunpromoted hot potassium carbonate solutions with solutions promotedwith diethanolamine (DEA) and solutions promoted with stericallyhindered amines. Both DEA and sterically hindered amines were found tobe very effective in increasing the rate of CO₂ absorption. Howeverequilibrium partial pressure of CO₂ is decreased after activator addedinto the carbonated solution. This means that it is more difficulty torelease CO₂ from activated solution than unactivated solution.

[0008] This invention provides a new method for gas liquid absorptionprocess. By introducing another liquid phase into original absorptionsystem to improving mass transfer resistence, the absorption rate isable to be increased significantly. More important, the increase ofabsorption rate does not cause any difficulty of regeneration.

SUMMARY OF THE INVENTION

[0009] A new method called phase enhanced gas-liquid absorption has beendeveloped. The absorption rate can be enhanced by adding another phaseinto original absorption system.

[0010] A system with three phase was studied. In the system, gas phasewas carbon dioxide. Two liquid phases were used. One was organic phase.Another was aqueous phase. By addition of organic phase into CO₂—aqueousphase absorption system, the absorption rate of CO₂ was increasedsignificantly. CO₂ finally accumulated in aqueous phase. Our experimentsproved that

[0011] (1) the absorption rate of carbon dioxide was enhanced by addingorganic phase into gas-aqueous phase system;

[0012] (2) organic phase played the role of transportation of gas solute(CO₂). Carbon dioxide finally accumulated into aqueous phase.

[0013] This invention provides a new technology for gas separation byaddition of organic phase into CO₂—aqueous phase absorption system. Byimproving mass transfer resistance in absorption process, thistechnology is able to lead to a more efficient, low cost absorptionprocess, and significantly reduce design constraints and costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a Schematic diagram of mass transfer model.

[0015]FIG. 2 is a Schematic diagram of the stirring cell in theapparatus of the invention.

[0016]FIG. 3 is a Schematic diagram of the apparatus used for separatinggas in accordance with the invention.

[0017]FIG. 4 shows a comparison of the absorption rate of CO₂ by organicphase+aqueous phase with aqueous phase. T=25° C.; P=1 atm; agitationspeed =250 rpm.

[0018]FIGS. 5A and 5B show a comparison of the absorption rates of CO₂by the organic phase and the aqueous phase. T=25° C.; P=1 atm; V=300 ml;agitation speed=250 rpm (5A); 106=rpm (5B)

[0019]FIGS. 6A and 6B show the relationship between the absorptionbetween the absorption rate of CO₂ with absorption time. T=25° C.; P=1atm; V=300 ml; agitation speed =250 rpm (6A); 106 rpm (6B)

DETAILED DESCRIPTION OF THE INVENTION

[0020] 1. Basic Concept

[0021] The definitions of absorption is a process which transfers one ormore components of a gas phase to a liquid phase in which it is soluble.The operation of absorption can be categorized on the basis of thenature of the interaction between absorbent and absorbate into followingtwo types, traditionally[ Perry, John H. Chemical Engineering Handbook;McGraw-Hill, Inc. 1963][Danckwerts, P. V. Gas-Liquid Reactions;McGraw-Hill, Inc. 1970]:

[0022] (1) Physical absorption. The component being absorbed in physicalabsorption is more soluble in the liquid absorbent than are the othergases with which it is mixed but does not react chemically with theabsorbent.

[0023] (2) Chemical absorption. Chemical absorption is characterized bythe occurrence of a chemical reaction between the gaseous componentbeing absorbed and a component in the liquid phase to form a compound.

[0024] (3) Phase enhanced absorption or Phase enhanced gas-liquidabsorption. The new concept we present here is “Phase enhancedgas-liquid absorption”. In our study, we found that adding organic phaseinto the gas-liquid (aqueous phase) absorption system is able to enhancethe absorption rate. We call this type of process as phase enhancedgas-liquid absorption. In phase enhanced gas-liquid absorption, morethan one liquid phases was involved in the absorption process. One ofthe liquid phase was called as carrying phase, in which gas solute willbe finally accumulated. Other liquid phase is known as transportationphase. The transportation phase only plays the rule for thetransportation of the gas solute from gas phase to carrying phase andfor the increase of absorption rate. In phase enhanced absorption, theabsorption rate is enhanced by the transportation phase.

[0025] (4) Mass Transfer Equation

[0026] Mass transfer equation for physical absorption can be expressed:

N _(A) =k _(L)(C _(A) *−C _(A))

[0027] where N_(A) is the rate of absorption per unit area of surface.k_(L) is the physical mass transfer coefficient. C_(A)* is theconcentration of dissolved gas corresponding to equilibrium with thepartial pressure of the gas at the interface between gas and liquid.C_(A) is the average concentration of dissolved gas in the bulk of theliquid.

[0028] For chemical absorption, the Mass transfer equation can beexpressed:

N _(A) =E _(R) k _(L)(C _(A) *−C _(A))

[0029] Here E_(R), enhancement factor, can be identified with the ratioof rate of absorption in the presence of reaction to the rate ofabsorption without reaction.

[0030] Similarly, for the phase enhanced absorption, the Mass transferequation can be expressed:

N _(A) =E _(P) k _(L)(C _(A) *−C _(A))

[0031] Here E_(p) can be identified with the ratio of rate of absorptionin the presence of another phase (transportation phase) to the rate ofabsorption without transportation phase.

[0032] (5) Mass Transfer Model for Phase Enhanced Absorption

[0033] By above definition, mass transfer of phase enhanced gas-liquidabsorption can be described as follows:

[0034] When gas contacts with liquid phases, gas is first absorbed bytransportation phase. The absorption is either physical or chemical. Thegas solute dissolved in the transportation phase passes through theinterface between transportation phase and carrying phase, and thenenters into carrying phase. In carrying phase, gas solute may exist intwo form, physical solubility or chemical combination. The function ofthe transportation phase is to deliver gas solute from gas phase tocarrying phase and to increase the absorption rate. With film theory,the mass transfer model of phase enhanced gas-liquid absorption can bedrawn as FIG. 1.

[0035] The pathway of gas solute from gas phase to carrying phase can bestated as follows:

[0036] (1) gas solute transfer from bulk of gas phase to the interfaceof gas-transportation phase.

[0037] (2) gas solute transfer from the interface of gas-transportationphase to the interface of transportation-carrying phase. In thetransportation phase, gas solute may react with the components intransportation phase or just simply physical solubility.

[0038] (3) gas solute at the interface of transportation-carrying phasetransfers into the bulk of carrying phase. Same as above, gas solute hastwo ways to deal with the components in carrying phase: one is physicalsolubility, i.e. without reaction between gas solute and the componentsin carrying phase; another is chemical combination, i.e. there isreaction between gas solute and the components in carrying phase.

[0039] 2. Experimental Apparatus and Method

[0040] The main experimental equipment was a stirring cell. Thestructure of the cell is shown in FIG. 2. The stirring cell is made ofglass with 100 mm inner diameter and 130 mm depth. Two agitating blades,one for liquid, one for gas, was driven by direct current motor. Theagitating speed was counted by a laser meter. The experimental apparatusis shown in FIG. 3. Carbon dioxide from cylinder 1 past through bufferbottle 3 and pressure stable tube 4. Gas flow rate was controlled andmeasured by rotating flow meters 5,8. Gas clean system consists of two Utubes. U tube 6 was filled by silicon gel. U tube 7 was filled by activecarbon. Gas was saturated with moisture by bottle 9 which contained thesame solution as that in stirring cell 14, and then absorbed in stirringcell 14. Gas was measured by foam film flow meter 10 and 12 before andafter reaction cell 14. After measurement, gas was released.

[0041] The absorption rate of carbon dioxide at time t was determined bythe difference of two flow rates, in and out of stirring cell, measuredby two foam film flow meters 10 and 12. As the results of themeasurement, the relationship of absorption rate r and time t would beobtained. Integration of absorption rate with time, r˜t, the totalamount of carbon dioxide absorbed into the liquid phase can be obtained.

[0042] The equilibrium concentration of carbon dioxide in carrying phasecan be simply determined by using ideal gas equation after carbondioxide released from carrying phase by sulfuric acid. The mass balanceof carbon dioxide from integration of r˜t data and the analysis ofcarrying phase should be matched well. If error higher than 5%, the datadiscarded.

[0043] The experiments were conducted at room temperature (25° C.) and 1(atm). Gas solute was carbon dioxide. The purity of carbon dioxide washigher than 99.9% two liquid phases were involved. One was organic phase(transportation phase), which played the role of transportation of gassolute CO₂. Another was aqueous phase, which was carrying phase. Theorganic phase was made up of Alamine 336, a mixture of C₈-C₁₂ tertiaryamine, and 2-ethylhexyl alcohol (1:1 ratio by volume). Aqueous phase wasthe solution of sodium formate (400 g/l). The absorption was operated atsemi-continuous basis. Carbon dioxide run through the absorptionapparatus continuously. The liquid was run batchwise. The absorptionsystem was agitated by two blades in agitation cell, one for liquid, onefor gas. The agitation speed and phase ratio (transportationphase:carrying phase) is specified in each experiment.

[0044] The method in accordance with the invention is illustrated on thebasis of following examples in which gas takes CO₂ for example. Inventedmethod can be applied to the separation and purification of carbondioxide from gas mixture. However, it is not limited to carbon dioxideabsorption process. The invented method also can be applied to other gasabsorption processes, such as SO₂, H₂S etc.

EXAMPLE 1 Absorption Rate was Enhanced by Adding Organic Phase Into theAbsorption System of CO₂-sodium Formate Aqueous Solution

[0045] Two experiments were designed to compare the rates of carbondioxide absorption (a) carbon dioxide was absorbed by 280 ml sodiumformate solution (400 g/l)+20 ml organic phase (50% Alamine 336 byvolume and 50% 2-ethylhexyl alcohol by volume); (b) carbon dioxide wasabsorbed by 300 ml sodium formate solution (400 g/l) directly.

[0046] In experiment (a), 280 ml sodium formate solution (400 g/L) and20 ml organic phase were added into agitating cell. The organic phaseexisted on the top layer. Liquid was agitated gently to avoid breakingthe organic layer. Carbon dioxide from gas phase has to pass through theorganic phase in order to enter into aqueous phase. In experiment (b),300 ml sodium formate solution (400 g/L) was placed into agitating cell.Both experiments operated under the same experimental conditions,temperature=25 ° C., pressure=1 atm, agitation speed=250 rpm, theconcentration of CO₂ was higher than 99.9%. The results are shown inFIG. 4. From FIG. 4, it can be seen that the absorption rate of carbondioxide in experiment (a) was higher than that in experiment (b). Thismeans that absorption rate of CO₂ by sodium formate aqueous solution wasincreased by adding organic phase into the system.

EXAMPLE 2 Organic Phase (Transportation Phase) Plays the Role ofTransportation of Carbon Dioxide from Gas Phase to Aqueous Phase

[0047] (1) Comparison of Absorption Rate Both by the Organic Phase andAqueous Phase

[0048] CO₂ absorption rate by aqueous phase and by organic phase wasmeasured individually at the same experimental condition, i.e.,temperature (25 ° C.), pressure (1 atm, 99.9% CO₂ gas used), agitationspeed (250 and 106 rpm). The experimental results are shown in FIG. 4.It can be seen from FIG. 4 that the absorption rate of CO₂ by 300 mlorganic phase (50% Alamine 336 by volume and 50% 2-ethylhexyl alcohol byvolume) much faster than that by 300 ml aqueous phase (the aqueoussolution of sodium formate 400 g/L). FIG. 5(a) was the result at theagitation speed of 250 rpm. FIG. 5(b) was the result at the agitationspeed of 106 rpm. It can be seen that the absorption rate of carbondioxide by organic phase was ten times more than that by aqueous phase.When aqueous phase and organic phase existed in the same absorptionsystem, the absorption of carbon dioxide by aqueous phase is able to beneglected as long as the surface area between gas and organic phase islarger than or equal to the surface area between gas and aqueous phase.

[0049] The relationship between the absorption rate of carbon dioxidewith absorption time is shown in FIG. 6. From FIG. 6, it is obvious thatcarbon dioxide reached equilibrium in organic phase much earlier thanthat in aqueous phase. So, the organic phase was saturated by carbondioxide much earlier than the aqueous phase when both in the same systemat the same time and the same experimental conditions.

[0050] The experimental results also proved that the mass transferresistance of CO₂ absorption by organic phase was much lower than thatby aqueous phase.

[0051] (2) Possibility of Carbon Dioxide to be Transferred from OrganicPhase to Aqueous Phase

[0052] To determine whether carbon dioxide absorbed in organic phase wasable to be transferred into aqueous phase, the following experiment wasdesigned. The calcium chloride was selected because the similarmechanism of reaction and mass transfer occurs as sodium formate aqueoussolution. Furthermore, calcium carbonate and calcium bicarbonate hadlittle solubility in water. The results were easy to be identified bythe white deposits if carbon dioxide was able to enter into aqueousphase from organic phase.

CaCl₂+2 CO₂+2H₂ O⇄Ca(HCO₃)₂↓+2HCl

[0053] As usual, 300 ml organic phase (50% Alamine 336 by volume and 50%2-ethylhexyl alcohol by volume) was placed into the agitated cell.Carbon dioxide was absorbed into the organic phase. CO₂ gas was stoppedafter the organic phase was saturated with carbon dioxide. Then calciumchloride solution with the concentration of 300 g/l was pored into theagitating cell immediately. The liquid was kept agitated at the rate of300 r/min. The experimental temperature was 25° C. Rest of theexperimental conditions were: pressure=1 atm, organic phase volume=300ml, volume of aqueous phase=80 ml. After about ten minutes of agitation,large amounts of white deposit appeared in the aqueous phase. Thisexperiment proved that carbon dioxide absorbed into organic phase wasable to be transferred into aqueous phase and further reacted with thesalt (calcium chloride in this case) in the aqueous phase. was keptagitated at the rate of 300 r/min. The experimental temperature was 25°C. Rest of the experimental conditions were: pressure=1 atm, organicphase volume=300 ml, volume of aqueous phase=80 ml. After about tenminutes of agitation, large amounts of white deposit appeared in theaqueous phase. This experiment proved that carbon dioxide absorbed intoorganic phase was able to be transferred into aqueous phase and furtherreacted with the salt (calcium chloride in this case) in the aqueousphase.

1. A method for gas separation by phase enhanced gas-liquid absorptioncomprising three phase: gas phase, organic phase and aqueous phase,wherein said organic phase is added into gas-aqueous phase absorptionsystem and resulted in an increase of absorption rate, and said organicphase is played the role of transportation of gas solute from gas phaseto aqueous phase.
 2. The method of claim 1, wherein said gas phase areCO₂, SO₂, H₂S or its mixture.
 3. The method of claim 1, wherein saidorganic phase are alkane, alcohol, ether, ester, amine or organiccompounds forming two phases with aqueous phase or their mixture.
 4. Themethod of clam 1, wherein the ratios of said organic phase to saidaqueous phase are 1 to 1-1000 by volume.
 5. The method of claim 1,wherein the gas solute content in said gas phase is about 0.001% toabout 100%.
 6. The method of claim 1, wherein said gas separation iscarried out at temperature from about 1° C. to about 500° C.
 7. Themethod of claim 1, wherein said gas separation is carried out under apressure from about 10⁻⁶ atm to about 10⁶ atm.
 8. The method of claim 1,wherein said aqueous phase include all types of aqueous solution. 9.Invented method can be applied to the separation and purification ofcarbon dioxide from gas mixture. However, it is not limited to carbondioxide absorption process. The invented method also can be applied toother gas absorption processes, such as SO₂, H₂S etc.